As seen by the aid of this fine telescope the spectrum of Uranus is found to be complete, 'no part being wanting, so far as the feebleness of its light permits it to be traced.' But there are six dark bands, or strong lines, indicating the absorptive action of the planet's atmosphere. One of these strong lines corresponds in position with one of the lines of hydrogen. Now it may seem at a first view that since the light of Uranus is reflected solar light, we might expect to find in the spectrum of Uranus the solar lines of hydrogen. But the line in question is too strong to be regarded as merely representing the corresponding line in the solar spectrum; indeed, Dr. Huggins distinctly mentions that 'the bands produced by planetary absorption are broad and strong in comparison with the solar lines.' We must conclude, therefore, that there exists in the atmosphere of Uranus the gas hydrogen, sufficiently familiar to us as an element which appears in combination with others, but which we by no means recognise as a suitable constituent (at least to any great extent) of an atmosphere which living creatures are to breathe.1 And not only must hydrogen be present in the atmosphere of Uranus, but in such enormous quantities as to be one of the chief atmospheric constituents. The strength of the hydrogen line cannot otherwise be accounted for. If by the action of tremendous heat all the oceans of our globe could be changed into their constituent elements, hydrogen and oxygen, it is probable that the signs by which an inhabitant of Venus or Mercury could recognise that such a change had taken place would be very much less marked than the signs by which Dr. Huggins has discovered that hydrogen exists in the atmosphere of Uranus. It will indeed be readily inferred that this must be the case, when the fact is noted that no signs whatever of the existence of nitrogen can be recognised in the spectrum of Uranus, though it is difficult to suppose that nitrogen is really wanting in the planet's atmosphere. Dr. Huggins also notes that none of the lines in the spectrum of Uranus appear to indicate the presence of carbonic acid. Nor are there any lines in the spectrum of Uranus corresponding to those which make their appearance in the solar spectrum when the sun is low down, and is therefore shining through the denser atmospheric strata. Most of these lines are due to the presence of aqueous vapour in our atmosphere, and it would seem to follow that if the vapour of water exists at all in the atmosphere of Uranus its quantity must be small compared with that of the free hydrogen.
1 Traces of hydrogen can nearly always be detected in the air, -but the quantity of hydrogen thus shown to be present is almost infinitesi-mally small compared with the amount of oxygen and nitrogen.
Admitting that the line seen by Dr. Huggins is really due to hydrogen - a fact of which he himself has very little doubt - we certainly have a strange discovery to deal with. If it be remembered that oxygen, the main supporter of such life as we are familiar with, cannot be mixed with hydrogen without the certainty that the first spark will cause an explosion (in which the whole of one or other of the gases will combine with a due portion of the other to produce water), it is difficult to resist the conclusion that oxygen must be absent from the atmosphere of Uranus. If hydrogen could be added in such quantities to our atmosphere as to be recognisable from a distant planet by spectroscopic analysis, then no terrestrial fires could be lighted, for a spark would produce a catastrophe in which all living things upon the earth, if not the solid earth itself, would be destroyed. A single flash of lightning would be competent to leave the earth but a huge cinder, even if its whole frame were not rent into a million fragments by the explosion which would ensue.
Under what strange conditions then must life exist in Uranus, if there be indeed life upon that distant orb. Either our life-sustaining element, oxygen, is wanting; or, if it exists in sufficient quantities (according to our notions) for the support of life, then there can be no fire, natural or artificial, on that giant planet. It seems more reasonable to conclude that, as had been suspected for other reasons, the planet is not at present in a condition which renders it a suitable abode for living creatures.
The St Paul's Magazine for October 1371.